The invention relates to processes for purifying industrial waste streams and more particularly to processes for treating waste solutions electrochemically to remove chromium and other heavy metal ions.
Chromium-containing solutions are used in a number of industries usually for chrome plating steels or for inhibiting corrosion in steel vessels. These solutions are acid solutions having various compositions depending upon the article to be treated. Hexavalent chromium in some form--such as sodium dichromate, sodium chromate, potassium dichromate, potassium chromate, chromium tri-oxide (chromic acid) and the like--is present in these solutions. Because hexavalent chromium is one of the most toxic chemicals to fish life, in even very minute concentrations on the order of one part per ten million, it must be substantially completely removed before treating solution waste waters containing chromium are discharged in a sewage system. Other metals and metal compounds which are found in such treating solutions can be precipitated out by suitably adjusting the pH of the waste waters. Hexavalent chromium, however is soluble at a pH of 0-14 and therefore special methods must be employed to remove it.
There have been various methods proposed to capture chromium before passing waste into a water body or sewer system. For example, U.S. Pat. No. 3,494,328 discloses metering stoichiometric amounts of a lead compound (such as lead nitrate) into a treating bath containing chromate ions to form an insoluble lead chromate. However, as is reported in U.S. Pat. No. 3,791,520, water containing a few parts per million hexavalent chromium was run through a bed containing a particulate insoluble lead compound such as lead oxide or hydroxide. Flow through the bed was very slow due to the formation of insoluble lead chromate which tended to cause packing of the bed. Channeling of the bed also occurred with upward flow of water therethrough with the result that the chromium in the water was not adequately removed.
U.S. Pat. No. 3,791,520 discloses contacting the chromium-laden water with a relatively water insoluble lead compound in conjunction with a porous, particulate matrix which prevents packing and channeling of the insoluble lead compound. The effluent from which the chromium has been removed is then run through a cation exchanger in the hydrogen form to remove any solubilized lead ion.
Removal of chromium from rinse waters by ion exchange methods is well known but is less than satisfactory because of the cost, the fact that the life of the ion exchange resin is short and can only be regenerated a few times before replacement, and because regeneration of the ion exchange resin yields a concentrated chromium solution which still presents a disposal problem.
A well known and commonly used method of removing chromium is to reduce chromium from the hexavalent state to the trivalent state with SO.sub.2 and then adjust the pH with calcium hydroxide to precipitate out the trivalent chromium together with relatively large quantities of calcium sulfate (gypsum) sludge. This method suffers from two serious deficiencies: (1) the gypsum has little commercial utility, and (2) the amount of trivalent chromium is usually too small and too contaminated by the gypsum to be of any commercial value. A further disadvantage of the SO.sub.2 method is that no free water is obtained that can be recycled for reuse in the metal treating system. Other methods of reducing hexavalent chromium employ sugar, wood, molasses and sawdust and are likewise unsatisfactory because of the production of large amounts of sludge having little or no commercial utility.
With regard to the removal of other metals, electrochemical designs have been tested for nickel or zinc removal in dilute acidic or neutral solutions. In H.S.A. Reactors Ltd., "Metal Finishing Report", Nov. 1977, pp. 26, H.S.A. Reactors Ltd. reported nickel reduction from 132 ppm to 14.5-22.0 ppm using a carbon fiber matrix cathode. This procedure has been termed "direct electrowinning". A fluidized bed "Chemelec" cell as reported in C. L. Lopez-Cacicedo, "The Recovery of Metals from Rinse Waters in `Chemelec` Electrolytic Cells", Trans. Inst. Metal Finishing, 53, 1975, pp. 74-77, reduced zinc from 523-550 ppm to 333-427 ppm and attempts at nickel reduction were simply termed "inconclusive". By contrast the "Swiss-roll" as reported in P. M. Robertson and N. Ibl, "Electrolytic Recovery of Metals from Waste Waters with the `Swiss-roll` Cell", J. Appl. Electrochem., 7, 1977, pp. 323-330, was used to selectively recover copper from copper/zinc and copper/nickel mixtures. It was reported that 99.9% of the copper was recovered with no detectable change in the conentration of the second metal.
These results show that complete removal of copper from acidic solutions is possible and significant nickel and zinc removal from dilute acidic solutions is possible when a flow-through cathode is used.
In accordance with the present invention, the direct electrowinning process may be used for the removal of copper and other heavy metals at the cathode and chromate removal at the anode.
Accordingly an object of this invention is to provide an electrochemical method and apparatus for the efficient removal of chromium and other heavy metals from dilute solutions.